Infrared optical systems with optical interference filters are frequently used to measure the gas concentrations of anesthetic gases such as inhalation anesthetics, carbon dioxide and laughing gas and also to determine the alcohol concentration in the breathing air of a person.
A gas analyzer known from DE 196 28 310 C2 comprises a radiation source, a measuring path receiving the gas sample and a detector, which is connected to an analyzing circuit. Different interference filters are brought one after another into the ray path by means of a rotating filter wheel. The transmission wavelengths of the filters are coordinated in the technical embodiment of the measuring arrangement with the absorption wavelengths of the gas components to be determined. To measure inhalation anesthetics, the transmission wavelengths of the filters are in a range of 8 μm to 12 μm, whereas those for laughing gas and carbon dioxide are in a range of 4 μm to 6 μm. The filter and transmission wavelength needed to measure drinkable alcohols is at 9.46 μm. Filters with a transmission wavelength of 3.4 μm and with a transmission wavelength range from 8.2 μm to 9.8 μm are advantageous for the measurement of acetone. Filters with a transmission wavelength range from 8.2 μm to 9.8 μm are likewise advantageous for the measurement of essential oils. Alcohol compounds and essential oils, for example, peppermint oil, can also be distinguished from each other besides the possibility to distinguish inhalation anesthetics from laughing gas and carbon dioxide. This makes it possible in case of patients whose alcohol concentration in the expired air is unphysiologically elevated to minimize the effect on the measuring results, which is associated therewith, in the determination of the gas concentration values of anesthetics and laughing gas.
The checking of the fitness of vehicle drivers to drive in connection with the monitoring of road traffic by measuring the breath alcohol concentration arises as an additional application from the possibility of distinguishing alcohols from essential oils. DE 10 2006 045 253 B3 describes a gas concentration-measuring device in which the absorption wavelengths are tuned with a dual-band Fabry-Perot interferometer. This has the advantage that the mechanically sensitive rotating filter wheel may be dispensed with in the arrangement and the device therefore has a more robust design against mechanical stresses. Another advantage is that the dual-band Fabry-Perot interferometer lets the wavelengths pass through in two spectral ranges. The transmission band of the dual-band Fabry-Perot interferometer is changed in this case by means of a control voltage such that both wavelength ranges are detected with one detector element and both wavelength ranges are analyzed simultaneously. To tune the wavelength ranges, the control voltage is applied electrically to the dual-band Fabry-Perot interferometer as a ramp function rising from a starting value to an end value and finally dropping again to the starting value. Two radiation sources, one source in the range of 4 μm to 6 μm, and another source in the range from 8 μm to 12 μm, are used as light sources. A broad-band radiation source, which emits light in the wavelength range from 4 μm to 12 μm, may be used as an alternative. By means of modulated radiation sources and a lock-in method, the dual-band Fabry-Perot interferometer is used in such an arrangement to simultaneously detect both wavelength ranges, to separate the measured values of the two wavelength ranges from one another and to analyze them. FIG. 1 of DE 10 2006 045 253 B3 shows the schematic diagram of a dual-band Fabry-Perot interferometer. A device for measuring gases, comprising a Fabry-Perot interferometer and a detector, is shown in US 2001015810A. A detector for detecting a plurality of spectral ranges is shown in EP 0536727 B1 in an embodiment as a multispectral sensor. The multispectral sensor comprises an array of optical elements for beam splitting in the form of a pyramid, whose reflection and transmission properties are spectrally different. The light is thus spectrally deflected selectively to the corresponding radiation-sensitive elements. A Fabry-Perot interferometer driven electrostatically, by means of a control voltage, is shown in DE 10226305 C1.
In terms of its transmission characteristic, the dual-band Fabry-Perot interferometer possesses the disadvantageous property of hysteresis of the transmitted wavelength when the control voltage is raised and lowered. This causes the association between the control voltage and the transmission wavelength to be different for the rising ramp of the control voltage and for the sloping ramp of the control voltage. Therefore, either only the rising part or the dropping part of the control voltage can be used for the analysis. An effective rate of measurement of 1 Hz is necessary for the application in the measurement of anesthetic gases and breath alcohol for a measurement resolution for each breath in inspiration and expiration. A repetition frequency of 2 Hz is necessary for this for running through a complete ramp of the control voltage for tuning two wavelength ranges of the dual-band Fabry-Perot interferometer when analyzing only one ramp of the control voltage. Modulation of the radiation source is necessary for the application of the lock-in method. To obtain sufficient spectral information in the received signal, a modulation frequency that is higher by a factor of at least 10 than the repetition frequency for tuning the dual-band Fabry-Perot interferometer is necessary for a robust technical embodiment. A distance of the modulation frequency by a factor of 3 is necessary in the practical and measuring technical embodiment for distinguishing two spectral ranges of interest from the spectral information for the two spectral ranges. This means, as a result, that a modulation frequency of 20 Hz is necessary for the first wavelength range of interest and a modulation frequency of 60 Hz is necessary for the second wavelength range of interest. When using a thermal radiation source, there is a significant decrease in amplitude with increasing modulation frequency above a modulation frequency of 30 Hz as a property of the thermal radiation source, which adversely affects the signal-to-noise ratio of the received signal, so that the signal is hidden by the noise level for the second wavelength range of interest and it can no longer be detected. This means that a value of 30 Hz is used as the highest modulation frequency for the second wavelength range of interest, which then leads, in conclusion, to a modulation frequency of 10 Hz for the first wavelength range of interest and results in a value of 0.5 Hz for the rate of measurement. Thus, the breathing gas concentration is not determined with a time resolution that would make it possible to distinguish the concentration values for inspiration and expiration.